The cunning plan is to tart it up with a new abstraction layer that will help developers implement support without heavy coding.
Most DirectX 12 games do not support Multi-GPU configurations because it requires a shedload of programming. Some games where it was done well have shown that DirectX 12 can handle multiple GPUs. They even managed to connect non-matching GPUs like the R9 Fury X and the GTX 980Ti to be used together.
According to OC3D the new abstraction layer is not available yet but Microsoft does state that they will be making a public release of this new toolset soon.